While polymeric materials are used for insulating a wide variety of electrical apparatus, they pose serious problems for high voltage applications in contaminated atmospheres where moisture or fog, together with salts, dust particles and ionic pollution, causes leakage currents to flow across the surface of the insulation. This current causes a rise in temperature with consequent moisture evaporation and ultimately dry band information. The electrical stress across these dry bands often exceeds the breakdown stress of the air-insulation interface, so that discharge or spark scintillation takes place. The spark temperature is extremely high, often 2000.degree. C or higher, and the heat produced thereby may be sufficient to cause degradation of the insulation surface with the ultimate formation of carbonaceous spots. These carbonaceous spots usually link up in dendritic fashion and the organic insulation fails by progressive creepage tracking.
Over the years, many solutions to this problem have been proposed. Perhaps the most effective has been the incorporation of hydrated alumina, preferably the trihydrate, in fairly substantial quantities into, for example, butyl rubber, epoxy resins, especially of the cycloaliphatic type, and, more recently, into ethylene-propylene rubbers as taught, for example, in U.S. Pat. Nos. 2,997,526; 2,997,527; and 2,997,528, the disclosures of which are incorporated herein by reference. It was found in practice that the polymeric materials containing large proportions of alumina trihydrate were substantially protected against tracking. However, in some cases, the materials still failed by tracking and further, in many cases, the materials failed by a gradual and progressive in-depth erosion or cratering of the insulation which occurs during over-voltage exposure. Further, the amount of alumina hydrate required to produce the anti-tracking effect is very high and is usually in the region of 50-90% by weight of the entire insulation. In the case of polymers that are shaped by molding or extrusion or used to make heat-recoverable articles, a content of alumina hydrate this high is undesirable because the high temperature used and/or the radiation employed in cross-linking causes loss of the hydrated water with accompanying development of porosity and the formation of voids leading ultimately to failure of the insulation. The high filler content is also undesirable because it is detrimental to certain mechanical properties of the polymer such as elongation.
Another solution to the tracking and erosion problem is disclosed in a copending commonly assigned application of Penneck et al, Ser. No. 434,126, filed Jan. 17, 1974. Penneck discloses forming an anti-tracking filler composition comprising a mixture of alumina hydrate and the oxides of transition elements, elements of the lanthanide series or of the non-transuranic actinide series. The composition is effective in preventing tracking and also functions to retard erosion. However, the oxides, such as iron oxide, are often highly colored which, in some cases, precludes their use due to environmental and/or aesthetic considerations. It has, therefore, been found that it would be desirable to form an anti-tracking and anti-erosion composition that would be even more effective in retarding the erosion rate of the polymer and that would also be a neutral color to allow coloring of the polymer with light gray or blue pigments.